N/A
N/A
N/A
The field of this invention is that of deepwater accumulators for the purpose of providing a supply of pressurized working fluid for the control and operation of equipment. Typical equipment includes, but is not limited to, blowout preventers (BOP) which are used to shut off the well bore to secure an oil or gas well from accidental discharges to the environment, gate valves for the control of flow of oil or gas to the surface or to other subsea locations, or hydraulically actuated connectors and similar devices. The fluid to be pressurized is typically an oil based product or a water based product with additives lubricity and corrosion protection.
Currently accumulators come in three styles and operate on a common principle. The principle is to precharge them with pressurized gas to a pressure at or slightly below the anticipated minimum pressure required to operate equipment. Fluid can be added to the accumulator, increasing the pressure of the pressurized gas and the fluid. The fluid introduced into the accumulator is therefore stored at a pressure at least as high as the precharge pressure and is available for doing hydraulic work.
The accumulator styles are bladder type having a balloon type bladder to separate the gas from the fluid, the piston type having a piston sliding up and down a seal bore to separate the fluid from the gas, and a float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of gas.
Accumulators providing typical 3000 p.s.i. working fluid to surface equipment can be of a 5000 p.s.i. working pressure and contain fluid which raises the precharge pressure from 3000 p.s.i. to 5000 p.s.i.
As accumulators are used in deeper water, the efficiency of conventional accumulators is decreased. In 1000 feet of seawater the ambient pressure is approximately 465 p.s.i. For an accumulator to provide a 3000 p.s.i. differential at 1000 ft. depth, it must actually be precharged to 3000 p.s.i. plus 455 p.s.i. or 3465 p.s.i.
At slightly over 4000 ft. water depth, the ambient pressure is almost 2000 p.s.i., so the precharge would be required to be 3000 p.s.i. plus 2000 p.s.i. or 5000 p.s.i. This would mean that the precharge would equal the working pressure of the accumulator. Any fluid introduced for storage would cause the pressure to exceed the working pressure, so the accumulator would be non-functonal.
Another factor which makes the deepwater use of conventional accumulators impractical is the fact that the ambient temperature decreases to approximately 35 degrees F. If an accumulator is precharged to 5000 p.s.i. at a surface temperature of 80 degrees F., approximately 416 p.s.i. precharge will be lost simply because the temperature was reduced to 35 degrees F. Additionally, the rapid discharge of fluids from accumulators and the associated rapid expansion of the pressurizing gas causes a natural cooling of the gas. If an accumulator is quickly reduced in pressure from 5000 p.s.i. to 3000 p.s.i. without chance for heat to come into the accumulator (adiabatic), the pressure would actually drop to 2012 p.s.i.
A fourth type accumulator has been developed which is one which is pressure compensated for depth, and is illustrated in the U.S. Pat. No. 6,202,753. This style operates effectively like a summing relay to add the nitrogen precharge pressure plus the ambient seawater pressure to the working fluid. This means that irrespective of the seawater depth (pressure), the working fluid will always have a greater pressure available for work by the amount of the nitrogen precharge.
This xe2x80x9cpressure compensatedxe2x80x9d style has numerous advantages in addition to the pressure compensation. It allows lower gas pressures with associated safety, eliminates the need to recharge the system for differing operational depths, and eliminates expensive mistakes in setting the charge pressures.
The pressure compensated type has exhibited two disadvantages. First it has required a relatively high pressure seal between the nitrogen chamber and the working fluid chamber. Very smooth seal surfaces are required to seal the nitrogen at relatively high pressures, and nitrogen still will tend to leak past the seals during dynamic movement. Secondly, there is some chance that the liquids will go past the seals and into the nitrogen chamber on one end and into the vacuum chamber on the opposite end and prevent effective performance of the accumulator.
The object of this invention is to provide a pressure compensated accumulator for deepwater ocean service which does not require a high pressure gas seal between a nitrogen chamber and an oil chamber.
A second object of the present invention is to provide a pressure compensated accumulator for deepwater ocean service which can prevent the accumulation of liquids in the vacuum chamber.
A third object of the present invention is to provide a pressure compensated accumulator for deepwater ocean service which can prevent the accumulation of liquids in the nitrogen chamber.